1. Field of the Invention
The present invention relates generally to golf balls incorporating peptizers. In particular, the invention relates to golf balls incorporating peptizers in their cores, intermediate layers, and covers. The present invention also relates to methods for manufacturing these golf balls.
2. Description of Related Art
Golf balls generally include a core and at least one cover layer surrounding the core. Golf balls can be classified as two-piece, multi-layer, or wound balls. Two-piece balls include a spherical inner core and an outer cover layer. Multi-layer balls include a core, a cover layer, and one or more intermediate (or mantle) layers. The intermediate layers can include multiple layers. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer, typically of balata material.
Material characteristics of the compositions used in golf ball layers are important in determining the durability and performance of the equipment. For example, the composition of a golf ball cover layer is important in determining the ball's durability, scuff resistance, speed, shear resistance, spin rate, feel, and “click” (the sound made when a golf club head strikes the ball). Various materials having different physical properties are used to make cover layers to create a ball having the most desirable performance possible.
For example, many modern cover layers are made using soft or hard ionomer resins, elastomeric resins, or blends of these. Golf ball cores generally incorporate polybutadiene rubbers, catalyzed by one of a number of known metals. Elastomeric resins used in golf ball covers include a variety of available thermoplastic or thermoset elastomers. Balata, polyurethane (cast, thermoset, and thermoplastic), and ionomers are the three most commonly used materials in this category.
Layers other than cover layers also significantly affect performance of a ball. The composition of an intermediate layer is important in determining the ball's spin rate, speed, and durability. The composition and resulting mechanical properties of the core are important in determining the ball's coefficient of restitution (C.O.R.), i.e., the ratio of the ball's post-impact to pre-impact speed, which affects ball speed and distance when hit, as well as core compression, i.e., a measure of the deflection on the surface of the ball when a standard force is applied. In addition to the performance factors discussed above, processability also is considered when selecting a formulation for a golf ball composition. These same considerations of durability and ease of manufacture are relevant for a wide variety of sports equipment.
Various materials having different physical properties are used to make sports equipment having the most desirable performance possible. One material generally cannot optimize all of the important properties for a particular piece of equipment. For golf balls, properties such as feel, speed, spin rate, resilience, and durability all are important, but improvement of one of these properties by the use of a particular material often can lead to worsening of another property. For example, ideally, a golf ball cover should have good feel and controllability, without sacrificing ball speed, distance, or durability. Despite the broad use of copolymeric ionomers in golf balls, their use alone in, for example, a ball cover can be unsatisfactory. A cover providing good durability, controllability, and feel would be difficult to make using only a copolymeric ionomer resin having a high flexural modulus, because the resulting cover, while having good distance and durability, also will have poor feel and low spin rate, leading to reduced controllability of the ball. Also, the use of particular elastomeric resins alone can lead to compositions having unsatisfactory properties, such as poor durability and low ball speed. With respect to golf ball cores, these cores are formulated to provide the highest possible C.O.R., regardless of their compression, to maximize ball distance when hit. Cores exhibiting low compression provide for improved ball feel, but also tend to exhibit reduced C.O.R.
Golf ball cores generally incorporate polybutadiene rubbers cross-linked with sulfur compounds, or peroxides along with zinc oxide and a metal salt of an acrylate, such as ZDA or ZDMA. These compositions provide for improved properties, however, even with the blending of materials, ideal properties have not been achieved in golf balls. For example, high C.O.R. is preferred because, as mentioned above, it provides for greater ball flight distance. Increasing loading levels of sulfur compounds, peroxides, or acrylate metal salts in the polybutadiene rubber used for a core composition is known to increase C.O.R. However, this also will lead to increased core compression, resulting in poorer ball feel. This relationship between C.O.R. and compression can be adjusted only to a limited extent using known activators, cross-linking agents, and co-cross-linking agents.
A variety of other materials, such as fillers and processing aids, have been used in making golf balls. For example, small amounts of chemical peptizers have been incorporated in golf ball cores. These peptizers generally are used in small amounts to accelerate the softening of polybutadiene rubber under the influence of mechanical force, heat, or a combination of these. Use of these peptizers allows for incorporation of a wider array of active chemical ingredients and fillers in a composition than in their absence. These peptizers also provide for easier processing of these compositions, as well as lower processing temperatures.
In view of the above, it is apparent that improved golf ball cores, intermediate layers, and cover layers that provide optimal performance and durability properties, while demonstrating ease of manufacture, as well as methods for making these cores, intermediate layers, and cover layers, are needed. The present invention fulfills these needs and provides further related advantages.